The present invention relates to the field of ophthalmic lenses. More particularly, the present invention relates to ophthalmic lens having a progressively varying focal power.
Middle age people frequently experience an inability to sharply focus when viewing close objects. This condition is called presbyopia and it results from a reduced or defective elasticity of the eye.
People suffering from presbyopia are typically prescribed multi-focal ophthalmic lenses having two or more spherical surfaces of different refractive power. These lenses are designed such that the power of one surface will permit proper focusing on nearby objects while the other surface or surfaces enable the wearer to focus objects at greater distances. A person suffering from presbyopia will initially wear lenses having two segments, known as bifocals. Later, as the eye further deteriorates, the person may require lens having three segments, called trifocals, to assist in focusing on objects at intermediate distances
Multi-focal lenses have been in use for great many years and are generally quite acceptable insofar as their ability to provide improved vision. There are several drawbacks of multi-focal lenses. For example, conventional multi-focal lenses have a sharp dividing line between the segments of the lens. As a result of the sharp dividing line, a person wearing the multi-focal lenses perceives a jump when scanning across the discontinuity. For many wearers, it is difficult to adjust to this sensation.
It is also for common for middle age people to experience an inability to clearly focus on objects that lie between those distances for which the various segments of the lens are designed to focus. This condition is referred to as having a reduced power of accommodation.
Finally, many people having reduced accommodation powers are hesitant to wear multi-focal lenses because of their belief that it detracts from their appearance and suggests to others that they are growing old. Although these people may require corrective spectacles, they will not regularly wear them, and thus are not only sacrificing good vision but are also creating a safety problem as, for example, driving without proper glasses.
Recognizing the inadequacies of conventional multi-focal lenses, progressively variable focal power lenses were developed. These lenses, which are also called progressive power lenses, are designed to provide multi-focal lens characteristics without any sharp discontinuity between the various portions of the lens. Specifically, progressive power lenses have a progressive power portion positioned between and merging into the distance and reading portions of the lens. Furthermore, the progressive portion is designed to have a continuously varying focal power beginning with a focal power equal to that of the distance portion where it merges with the distance portion and ending with a focal power equal to that of the reading portion where it merges into the reading portion.
The distance, reading, and progressive powers portions constitute the functional zone of the lens. The remaining area of the lens constitutes the peripheral zone, which does not provide proper corrective power for the wearer. The peripheral zone typically contains aberrations that interfere a wearer's comfort.
The basic patent for this type of lens was issued in 1915 (U.S. Pat. No. 1,143,316) and a more refined version was issued in 1924 (U.S. Pat. No. 1,518,405). These patents primarily related to the functional zone of the lens. Numerous patents have subsequently issued that provide different formulations for the peripheral zone, aiming at reducing aberrations or relating to manufacturing methods or both.
Two aberrations in the peripheral zone that have a major impact on the wearer's comfort are astigmatism and distortion of horizontal and vertical lines. The physical and mathematical properties of these aberrations are well understood by those skilled in this field, and it is recognized that due to physical requirements which have to be fulfilled in the functional zone, these aberrations cannot be eliminated. This limits the design objective to the development of a surface geometry having particular distribution of aberrations in the peripheral zone so as to provide the best possible comfort to the wearer.
Particularly disturbing is the distortion of horizontal and vertical lines in the peripheral zone. Also, the unavoidable astigmatism aberration is particularly offending in the zone adjacent to the distance portion as it affects the near peripheral vision during distance viewing. Another undesirable place for this aberration is the lowest portion of the peripheral zone because it tends to distort true ground level perception. Typically, the more recent U.S. patents, such as U.S. Pat. Nos. 3,687,528, 3,2910,691, 4,055,379; and 4,062,629, describe the lenses that are all aimed at providing the best solution for the above described conditions.
Another patent that addresses reduction of aberrations is Kitchen et al., U.S. Pat. No. 4,307,945. The Kitchen et al. lens exhibits many improvements over prior art lenses. The lens reduces distortion of horizontal and vertical lines in the temporal peripheral zone below detectable levels. The lens also reduces the astigmatic aberration in the area adjacent to the distance portion. In addition, the lens significantly reduces astigmatic aberration in the lower part of the temporal peripheral zone. The design of the lens also allows the above described benefits to be retained when the lens is rotated to accommodate the left or right eye.